RESIDUE THREONINE-149 OF THE SALMONELLA-TYPHIMURIUM CYSB TRANSCRIPTION ACTIVATOR - MUTATIONS CAUSING CONSTITUTIVE EXPRESSION OF POSITIVELY REGULATED GENES OF THE CYSTEINE REGULON

In both Salmonella typhimurium and Escherichia coli, CysB is a LysR fa mily transcriptional activator, which regulates genes of the cysteine regulon. Transcription activation of cys genes also requires an induce r, N-acetyl-L-serine, and cysB mutants that do not require inducer are termed constitutive, i.e. cysB(c). After finding that two independent ly isolated cysB(c) mutants are substituted at amino acid residue thre onine-149 (T149), we isolated the other 17 single-amino-acid substitut ions by site-directed mutagenesis. Of the 19 mutant alleles, 11 suppor ted normal growth on sulphate, and nine of these were cysB(c). Four ot her mutants were 'leaky' cysB(+), and four were cysB(-). Insertions of up to 14 amino acids were also tolerated at T149, and two of three su ch mutants were cysB(c). An allele containing a TAG translation termin ator at codon 149 had no detectable function in a Delta cysB strain, b ut gave a constitutive phenotype when introduced into either wild-type S. typhimurium or the E. coli strain NK1, which contains a cysB(-) mu tation in a predicted helix-turn-helix region that interferes with spe cific binding of CysB to DNA and with autoregulation of cysB. The pept ide encoded by the T149ter allele is proposed to interact with the wil d-type CysB peptide or with the NK1 mutant peptide to form hetero-olig omers that do not require N-acetyl-L-serine for cys gene activation.